This application relates to a pulse width modulation control for a suction valve that allows for continuous and precise capacity adjustment to be provided by a refrigerant system in efficient and cost effective manner, and wherein compressor temperature is monitored to determine an optimum duty cycle for the pulse width modulation method from performance, comfort and reliability perspectives.
Refrigerant systems are utilized in many applications such as, for example, condition an indoor environment or refrigerated space. For instance, air conditioners and heat pumps are used to cool and/or heat the air entering an environment. The cooling or heating load in the conditioned environment may change with ambient conditions, internal thermal load generation, and as the temperature and/or humidity levels demanded by an occupant of the environment or requirements for the conditioned space are varied. Therefore, the refrigerant system operation and control have to adequately react to these changes in order to maintain stable temperature and humidity conditions within the environment, while preserving functionality, performance and efficiency as well as sustaining reliable operation.
One method that is known in the prior art to assist in the adjustment of capacity provided by a refrigerant system is the use of a pulse width modulation control. It is known in the prior art to apply a pulse width modulation control to cycle a suction valve at a certain rate for controlling the flow of refrigerant to a compressor, to in turn adjust refrigerant system capacity. Since the pulse width modulation valve is typically cycled between fully open and fully closed (or nearly fully closed) positions, minimal additional throttling or other noticeable performance losses are imposed during such part-load operation. By limiting the amount of refrigerant flow passing through the compressor, the capacity can be reduced to a desired level below a full-load capacity (approximately down to 5% of the total capacity) of a refrigerant system to precisely match the thermal load in a conditioned environment.
One problem raised by pulse width modulation of a suction valve is that a flow of refrigerant delivered into the compressor suction port may be significantly reduced. In many compressor designs, the suction refrigerant passes over the motor, to cool the motor. If the amount of refrigerant flowing through the compressor suction port is significantly reduced, it may not adequately cool the motor. The motor temperatures may increase dramatically and exceed a specified limit that in turn may lead to permanent motor damage and catastrophic failure. Moreover, since a lower amount of refrigerant is relied upon to cool the motor, that refrigerant can become excessively hot and may transfer this heat to other compressor components, overheating these components, including oil lubricating the compressor elements, which is highly undesirable. Also when compressor operates in a pulse width modulation mode, during the portion of the cycle when the pulse width modulation valve is closed or nearly closed, the operating pressure ratio can reach very high values. High pressure ratio operation coupled with excessive motor heat can lead to high discharge temperatures at the compressor discharge or within the compression elements. Thus, if the pulse width modulation technique is setup to cycle through relatively long periods of a suction valve being closed or nearly closed, the compressor components, oil and refrigerant can become extremely hot, leading to potential compressor reliability problems and nuisance shutdowns. Additionally, thermal inertia of a refrigerant system may not be sufficient enough to overcome and prevent temperature and humidity variations in a conditioned environment, causing occupant discomfort or hampering refrigeration.
On the other hand, if the valve is cycled too frequently to minimize the upper temperature excursions, the risk of suction valve failure may increases due to the extensive cycling, as well as secondary instability effects may propagate throughout the system interfering with its proper functionality.
Consequently, there is a need for a method to control a duty cycle for a pulse width modulation valve to eliminate all undesired phenomena mentioned above.